This invention relates to a rotary atomizer device for spraying a liquid coating material and more particularly to a rotary atomizer device wherein high electrostatic charge is transferred from a power supply to an improved high speed atomizer cup secured to a shaft driven by an air turbine motor. The atomizing cup has conductive pathways for transferring electrostatic energy to the coating material without igniting the propane used in the FM 7260 test. The cup is secured to the turbine shaft to facilitate quick demounting of the cup for cleaning or replacement. The rotary atomizer can be mounted to a robot and connected to a liquid supply by an elongated spiral passageway.
Rotary atomizers are a type of liquid spray coating device which includes an atomizer head rotatable at high speed (typically 10,000-45,000 revolutions per minute) by an air turbine motor to apply liquid coating material, such as paint, in atomized form onto the surface of a workpiece. The atomizer head is usually in the form of a disc or cup which includes an interior wall that defines a cavity and terminates in an atomizing edge. Liquid coating material delivered to the interior of the cup flows outwardly under centrifugal force along the interior wall of the cup and is expelled radially outward from the peripheral edge of the cup to form a spray pattern of atomized droplets of coating material. To improve the transfer efficiency of the coating process, an electrostatic charge is imparted to the coating material so that the pattern of atomized coating material is attracted to an electrically grounded workpiece.
An example of an electrostatically charged rotary atomizer is disclosed in commonly assigned U.S. Pat. No. 4,887,770 (""770) to Wacker et al., which is expressly incorporated herein in its entirety by reference.
Prior to the ""770 patent, one of the hazards associated with the use of the conductive atomizing cup was the possibility of operator shock or ignition of combustible coatings because of the high voltage at which the cups were maintained. For example, as disclosed in U.S. Pat. No. 4,369,924, a charge is transferred through a turbine shaft from a power supply to the rotary atomizer cup. Since, both the cup and the entire rotary atomizing housing were metal and were charged to a high voltage, there is a significant safety hazard since the atomizer carries sufficient charge to severely shock an operator. Therefore, protective fences and interlocks have to be installed around the atomizer.
The ""770 patent, listed before, discloses a low capacitance, rotary atomizer which, while electro-statically charging the coating paint at the rotary atomizer cup, does not store sufficient charge to present a shock hazard and therefore does not have to be protected by fences and safety interlocks. To charge the atomizer in the ""770 patent, external electrode probes (462) direct the charge into the cup (20). Still, under certain strict testing conditions, a charge on the equipment could still cause some safety concerns.
Another problem associated with prior art rotary atomizers is that the rotary atomizer cups have not been easy to disassemble and clean. For example, in U.S. Pat. No. 4,838,487, a deflecting member (28) is held in place against atomizing bell (10) by spacers (36). However, in operation, dried paint can collect on the front surface (30) of the deflector member. Then, the flow of paint across the front surface with the dried paint has a tendency to form an irregular coating on the part being sprayed.
Still another problem associated with the prior art electrostatic spray guns was related to the isolation of the spray gun with the liquid supply. One solution, as described in U.S. Pat. No. 4,139,155 to Hastings was to provide a spiral conduit in the flow passage.
It is an object of the present invention to provide an improved electrostatic rotary atomizing spray device as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
It is another object of the present invention to provide a rotary atomizer device for spraying a liquid coating and method of operating same wherein an improved rotary cup has a plurality of conductive pathways for transferring electrostatic energy to the paint without igniting the propane used in the FM 7260 test.
Still another object of the present invention is to provide a rotary atomizer device for spraying a liquid coating and method for assembling the device the atomizing head or cup can be easily removed from the atomizer device for cleaning.
It is still a further object of the present invention to provide an apparatus and method for transferring charge to a high speed atomizer head or cup through a charge ring mounted to the front of the rotary atomizer housing so that the charge is dissipated to prevent the need for protecting an operator from being shocked.
Still another object of the present invention is to provide an access hole in the charge ring for insertion of a tool to restrain free rotation of the turbine shaft to which the atomizer cup is secured to facilitate quick demounting of the atomizer cup for cleaning or replacement.
Yet another object of the present invention is to provide an additional electrode in the electrical circuit for transferring charge from the power supply to the atomizer cup through a charge ring, wherein the additional electrode is disposed in the access hole of the charge ring for presenting a low voltage in the vicinity of the access hole that provides a small spark which is insufficient to ignite the propane used in the FM 7260 test.
Still another object of the present invention is to mount the rotary atomizer with the improved charge ring and rotary cup to a robot and connected to the liquid supply control valve by an elongated spiral passageway to increase the electrical resistance between the atomizer and the liquid supply so that a very small electrical charge, if any, will be present in the passageway carrying paint to the rotary cup.
According to an embodiment of the invention, a rotary atomizer cup of an electrostatic, liquid spray, rotary atomizer has several elongate conductive pathways which are embedded in the body of the cup that is formed of a non-conductive material. Each conductive pathway has one end exiting an outer surface at a rear end of the cup for receiving the electrical charge from the power supply. Each conductive pathway has another end exiting an inner surface at a front end of the cup. The conductive pathways convey at electrical charge from the power supply to liquid (paint) particles passing through the atomizer cup. A number of conductive extensions are embedded in a frustroconical front portion of the cup. Each conductive extension has a first end portion contiguous with the inner surface, exiting end of selected ones of the conductive pathways and a second opposite end portion exiting an outer surface of the frustroconical front portion of the cup. The conductive extensions also have a second opposite end portion exiting an inner surface of the frustroconical front portion of the cup.
According to an aspect of the invention, an annular charge ring mounted to the front portion of the atomizer is configured to accommodate the atomizer cup with the conductive pathways and extensions. The charge ring has an access hole for facilitating insertion of a tool for restraining free rotation of the turbine shaft to which the atomizer cup is secured to facilitate quickly demounting the atomizer cup for cleaning or replacement. An electrode disposed within the access hole and electrically connected to the electrical circuit presents a low voltage in the vicinity of the access hole for providing a small spark that is insufficient to ignite the propane used in the FM 7260 test. In order that the access hole does not compromise the air-tightness of the atomizer housing, a duckbill valve is provided in the access hole.